Ultraviolet Raman Spectroscopy Research Articles

Low threshold field emission from high-quality cubic boron nitride films

Field emission performance of materials with mixed sp2/sp3 phases often depends upon the phase composition at the surface. In this study, the emission performance of high-quality cubic boron nitride (cBN) films is studied in terms of phase purity. Thick cBN films consisting of micron-sized grains are prepared from boron trifluoride gas by chemical vapor deposition in a plasma jet and an inductively coupled plasma. Both the bulk and surface phase purities as well as crystallinities of cBN evaluated by visible and ultraviolet Raman spectroscopy, glancing-angle x-ray diffraction, and x-ray photoelectron spectroscopy are the highest when the film is deposited in a plasma jet under an optimized condition. The emission turn-on field decreases with increasing the phase purity, down to around 5 V/μm for the highest cBN purity, due to the larger field enhancement, while it is higher than 14 V/μm without cBN (sp2-bonded hexagonal BN only). The results indicate that the total field enhancement for the high phase purity film is governed by the internal field amplification related to the surface coverage of more conductive cBN, rather than the external one related to the surface topology or roughness.

Nanopatterning of ultrananocrystalline diamond nanowires

We report the fabrication of horizontally aligned ultrananocrystalline diamond (UNCD) nanowires (NWs) via two different approaches. First, with the top-down approach by using electron beam lithography (EBL) and reactive ion etching (RIE) with a photo resist layer as an etch mask. Using this approach, we demonstrate fabrication of 50 µm long UNCD NWs with widths as narrow as 40 nm. We further present an alternative approach to grow UNCD NWs at pre-defined positions through a selective seeding process. No RIE was needed either to etch the NWs or to remove the mask. In this case, we achieved UNCD NWs with lengths of 50 µm and smallest width of 90 nm respectively. Characterization of these nanowires by using scanning electron microscopy (SEM) and atomic force microscopy (AFM) shows that the UNCD NWs are well defined and fully released, with no indication of residual stress. Characterization using visible and ultraviolet (UV) Raman spectroscopy indicates that in both fabrication approaches, UNCD NWs maintain their intrinsic diamond structure.

Room Temperature Photoluminescence and Ultraviolet Raman Characterization of Boron Implanted Silicon under Various Laser Annealing Conditions

Room temperature photoluminescence (RTPL) and ultraviolet (UV) Raman spectra from p−-Si wafers and low-energy, low-dose boron (B) implanted n−-Si wafers, annealed under various laser power densities, were measured. The RTPL intensity from implanted wafers, increased with increasing laser power density. UV Raman spectra showed significant, sudden changes in their shape and intensity around the surface melting condition. The sheet resistance of the implanted wafers was reduced as the laser power density decreased. Steep increase of RTPL intensity and steep reduction of the sheet resistance was measured from the B+ implanted n−-Si wafers above surface melting laser annealing conditions. A strong inverse correlation between RTPL intensity and sheet resistance was found for implanted wafers. The RTPL intensity, UV Raman spectra and sheet resistance of p−-Si reference wafers behaved quite differently compared to implanted wafers. The RTPL and UV Raman spectroscopy are very promising monitoring techniques for implant annealing processes.

Mechanical Stress Evaluation of Si Metal–Oxide–Semiconductor Field-Effect Transistor Structure Using Polarized Ultraviolet Raman Spectroscopy Measurements and Calibrated Technology-Computer-Aided-Design Simulations

Mechanical Stress Evaluation of Si Metal–Oxide–Semiconductor Field-Effect Transistor Structure Using Polarized Ultraviolet Raman Spectroscopy Measurements and Calibrated Technology-Computer-Aided-Design Simulations

Mechanical Stress Evaluation of Si Metal–Oxide–Semiconductor Field-Effect Transistor Structure Using Polarized Ultraviolet Raman Spectroscopy Measurements and Calibrated Technology-Computer-Aided-Design Simulations

The mechanical stresses in Si metal–oxide–semiconductor field-effect transistors (MOSFETs) were evaluated by polarized UV Raman spectroscopy measurements and stress simulations. To calibrate stress parameters of the materials used in the Si MOSFETs, we compared measured and simulated Raman frequency shifts on the cleaved Si(110) surfaces of the MOSFETs. Consequently, we extracted intrinsic stress values of -400 MPa for a SiO2, -200 MPa for polycrystalline Si (poly-Si), 700 MPa for Ni silicide, 1250 MPa for a SiN tensile stress liner, and -3500 MPa for a SiN compressive stress liner by finding good agreement between the measured and simulated Raman shift distributions. To verify our stress simulation, we investigated the source/drain width dependences of Raman frequency shifts near the channel regions of Si MOSFETs by top-view Raman measurements. The calculated Raman frequency shifts agreed well with the results of polarized Raman measurements in terms of not only relative tendencies but also absolute Raman shift values.

Comparison of the Effects of Biodiesel and Mineral Diesel Fuel Dilution on Aged Engine Oil Properties

Dilution of engine oil occurs when fuel is injected late in the combustion cycle to regenerate the diesel particulate filter used for trapping particulate emissions. Fuel dilution reduces oil viscosity and the concentration of engine oil additives, potentially compromising lubricant performance. Biodiesel usage may compound these issues due to its oxidative instability, and its higher boiling point compared to mineral diesel potentially causes it to concentrate more in the oil sump. In this work, different amounts of mineral diesel and biodiesel (soy methyl ester, SME) were combined with 15W-40 CJ-4 diesel engine oil in laboratory oil aging experiments. Fuel was added and oil samples were withdrawn at periodic intervals. The oils were analyzed using typical oil analysis procedures to determine their condition, and wear evaluations under boundary lubricating conditions were determined using a high-frequency reciprocating rig (HFRR). Results showed that fuel dilution accelerated engine oil degradation, with biodiesel having a larger effect. However, friction remained unchanged with dilution, and wear actually decreased for fuel-diluted oils after 48 h of aging compared to aging without fuel dilution. Examination of the tribofilms by ultraviolet (UV) and visible Raman spectroscopy as well as Auger electron spectroscopy showed that additional carbon-containing components were present on tribofilms formed from fuel-diluted oils. These fuel-derived components may be responsible for the decreased wear observed.

Ferroelectricity in nonstoichiometric SrTiO3 films studied by ultraviolet Raman spectroscopy

Homoepitaxial Sr1+xTiO3+δ films with −0.2≤x≤0.25 grown by reactive molecular-beam epitaxy on SrTiO3 (001) substrates have been studied by ultraviolet Raman spectroscopy. Nonstoichiometry for strontium-deficient compositions leads to the appearance of strong first-order Raman scattering at low temperatures, which decreases with increasing temperature and disappears at about 350 K. This indicates the appearance of a spontaneous polarization with a paraelectric-to-ferroelectric transition temperature above room temperature. Strontium-rich samples also show a strong first-order Raman signal, but the peaks are significantly broader and exhibit a less pronounced temperature dependence, indicating a stronger contribution of the disorder-activated mechanism in Raman scattering.

Measuring the electronic properties of single-walled carbon nanotubes with adsorbed porphyrins using optical transitions

The dielectric constants of diverse media surrounding single-walled carbon nanotubes (SWCNTs) were probed using photoluminescence (PL) excitation maps of porphyrin/SWCNT aqueous suspensions. The excitation and emission maxima of the nanotubes in these maps were used to probe the dielectric constant variation and doping originated from the porphyrin molecules. The net dielectric constant was calculated for the surrounding medium for each nanotube index and porphyrin isomer. The spread of the dielectric constant values calculated from the data for each (n, m) nanotube chiral index is interpreted on the basis of selective adsorption by each (n, m) nanotube, for each porphyrin isomer. Ultraviolet (UV) Raman spectroscopy corroborates the doping process through the shift of a G band around 1608 cm-1

Few-layer epitaxial graphene grown on vicinal 6H–SiC studied by deep ultraviolet Raman spectroscopy

Few layer epitaxial graphenes (1.8–3.0 layers) grown on vicinal 6H–SiC (0001) were characterized by deep ultraviolet Raman spectroscopy. Shallow penetration depth of the probe laser enabled us to observe G-peak of graphene without subtraction of the SiC substrate signal from observed spectra. The G-peak was greatly shifted to higher frequency compared to that of graphite due to in-plane compressive stress deriving from the substrate. The frequency shift decreased with the number of graphene layers because of stress relaxation from layer to layer. Our experiment suggests that the stress is completely relaxed within five to six graphene layers.

Mobility and Velocity Enhancement Effects of High Uniaxial Stress on Si (100) and (110) Substrates for Short-Channel pFETs

An experimental study of mobility and velocity enhancement effects is reported for highly strained short-channel p-channel field-effect transistors (pFETs) using a damascene-gate process on Si (100) and (110) substrates. The relationship between the mobility and the saturation velocity of hole under a compressive stress over 2.0 GPa is discussed. The local channel stress of 2.4 GPa is successfully measured with ultraviolet-Raman spectroscopy for the 30-nm-gate-length device with top-cut compressive-stress SiN liner and embedded SiGe. Mobility and saturation-velocity enhancements of (100) substrate are larger than those of (110) under the high channel stress. In consequence, the saturation current on (100) is larger than that on (110) for the pFETs with higher channel stress and shorter gate length. Moreover, the large enhancement rate of saturation velocity to mobility by the uniaxial stress suggests high injection velocity for the pFETs with the stressors since the high channel stress is induced near the potential peak of the source by using the damascene-gate technology.

Channel stress measurements of 45 nm node transistors with embedded silicon-germanium source and drain using ultraviolet Raman spectroscopy

We report the use of ultraviolet Raman spectroscopy to measure the average channel stress for 45 nm node transistors with embedded silicon-germanium (SiGe) source and drain. Direct probing of the channel for stress measurements was made possible by removing the polycrystalline silicon gate using a simple and cost-free approach. We demonstrate the feasibility of this method for measuring channel stress of dense transistor structures with varying pitch lengths, with ∼80 nm SiGe selectively grown in the source and drain regions.

Direct Measurement of MOSFET Channel Strain by Means of Backside Etching and Raman Spectroscopy on Long-Channel Devices

Measuring strain in long-channel MOSFETs on silicon-on-insulator (SOI) and strained-SOI platforms is demonstrated using ultraviolet (UV) Raman spectroscopy. Removal of the Raman inactive strain-inducing metallization layers is avoided by etching trenches under transistors without mask alignment in order to expose the channel region. The technique is shown to be repeatable and does not alter the initial strain state in the channel. The applicability of this technique to short-channel transistors is also discussed.

Solution Structures and Ultraviolet Raman Spectra of Purines Reveal Systematic Shifts with Change in Protonation State

Purines and pyrimidines that form the nucleobases in normal and modified DNA and RNA are known to exist in several tautomeric and ionization states. The relative populations of tautomers in solution and their precise structure are highly sensitive to ring substitution and pH. Knowing the structures in different environments is essential to understand the detailed chemistry of purine modifying enzymes such as ribosyl transferases and DNA glycosylases. We used ultraviolet Raman spectroscopy in resonance with nucleobases and exhaustive DFT calculations to reliably identify solution structures of several nucleic acid enzyme substrates at different pH and established their vibrational signatures. We find that the DNA damage lesion 8oxoguanosine is in the diketone form at physiological pH. At high pH, the anion is formed via deprotonation at N1 while the enolic form was not detected. Hypoxanthine exists as a mixture of the N7H and N9H forms though xanthine exists as a single diketone tautomer in solution. Xanthine is neutral at pH 7 but pKa changes accompany formation of the corresponding mononucleotide which is found to exist predominantly in the deprotonated form at this pH. The vibrational spectra show systematic shifts correlated with protonation state with an overall reduction in the wavenumbers of the purine ring vibrations upon deprotonation. The spectral signatures of purines established here can be used to selectively observe nucleic acid-protein interaction.

The optical properties of vertically aligned ZnO nanowires deposited using a dimethylzincadduct

The optical properties of zinc oxide nanowires are critically influenced bythe growth process. Herein, we describe a metal–organic chemical vapourdeposition (MOCVD) process for the growth of ZnO nanowires with improvedoptical properties. A tetrahydrofuran adduct is used to control the reactivity ofdimethylzinc to enable this. Vertically aligned zinc oxide nanowires have beengrown on Si(111) substrates by liquid injection MOCVD, using a solution of[Me2Zn(tetrahydrofuran)] in thepresence of oxygen. The ZnO morphology becomes nanowire-like in a narrow temperature range centredabout 500 °C. Above and below this temperature range, the ZnO is deposited in the form of polycrystallinefilms. The ZnO nanowires grow from a polycrystalline nucleation layer, with the (0002)c-axis parallelto the Si⟨111⟩ substrate orientation. High-resolution electron microscopy reveals a highly crystallinenanowire microstructure. Resonance enhanced ultraviolet Raman spectroscopy shows thatthe ratio of first- and second-order longitudinal optic modes is commensurate withelectron–phonon coupling effects observed previously in ZnO nanostructures.Photoluminescence exhibits intense near band-edge emission with a full width athalf-maximum of 110 meV at room temperature and shows negligible defect-related visibleemission.

Ferroelectricity in UltrathinBaTiO3Films: Probing the Size Effect by Ultraviolet Raman Spectroscopy

We demonstrate the dramatic effect of film thickness on the ferroelectric phase transition temperature Tc in strained BaTiO3 films grown on SrTiO3 substrates. Using variable-temperature ultraviolet Raman spectroscopy enables measuring Tc in films as thin as 1.6 nm, and a film thickness variation from 1.6 to 10 nm leads to Tc tuning from 70 to about 925 K. Raman data are consistent with synchrotron x-ray scattering results, which indicate the presence of 180 degrees domains below Tc, and thermodynamic phase-field model calculations of Tc as a function of thickness.

Local Structure of Nanocrystalline Diamond - Evolution with Thermal Treatments

The modifications of the sp2 carbon phases in CVD nanocrystalline diamond with 25 nm grain size are analysed by ultra-violet source Raman spectroscopy, Fourier transformed infra-red spectroscopy (FTIR) and and X-ray photoelectron spectroscopy (XPS), according to different thermal annealing treatments. Hydrogen concentration is measured by secondary ion mass spectrometry. The hydrogen outdiffusion of the sample as a function of temperature is recorded by mass spectrometry. The Raman spectra show evolution of the signal of the two G and D bands at 1360 and 1530 cm-1 related to more or less disordered sp2 phases, and of the 1150 cm-1 peak attributed to acetylenic chains located at grain boundaries. Thermal annealing breaks the short distance order in sp2 phases. FTIR and XPS analysis show that the sp2 CHx bonds are the less stable and that the superficial sp3 diamond carbon bonds recover as a result of the hydrogen departure.

Visible, near‐infrared, and ultraviolet laser‐excited Raman spectroscopy of the monocytes/macrophages (U937) cells

AbstractRaman spectra of the monocytes were recorded with laser excitation at 532, 785, 830, and 244 nm. The measurements of the Raman spectra of monocytes excited with visible, near‐infrared (NIR), and ultraviolet (UV) lasers lad to the following conclusions. (1) The Raman peak pattern of the monocytes can be easily distinguished from those of HeLa and yeast cells; (2) Positions of the Raman peaks of the dried cell are in coincidence with those of the monocytes in a culture cell media. However, the relative intensities of the peaks are changed: the peak centered around 1045 cm−1 is strongly intensified. (3) Raman spectra of the dead monocytes are similar to those of living cells with only one exception: the Raman peak centered around 1004 cm−1 associated with breathing mode of phenylalanine is strongly intensified. The Raman spectra of monocytes excited with 244‐nm UV laser were measured on cells in a cell culture medium. A peak centered at 1485 cm−1 dominates the UV Raman spectra of monocytes. The ratio I1574/I1613 for monocytes is found to be around 0.71. This number reflects the ratio between proteins and DNA content inside a cell and it is found to be twice as high as that of E. coli and 5 times as high as that of gram‐positive bacteria. Copyright © 2009 John Wiley & Sons, Ltd.

Probing the Interaction of Magnetic Iron Oxide Nanoparticles with Bovine Serum Albumin by Spectroscopic Techniques

The interaction of magnetic iron oxide nanoparticles (MNPs) with bovine serum albumin (BSA) was investigated by fluorescence (FL), ultraviolet visible (UV-vis) absorption, Raman, and circular dichroism (CD) spectroscopy. Results indicated that MNPs quench BSA FL mainly by a static quenching mechanism. The FL quenching constants KSV were obtained as 2.44x10(8), 2.41x10(8), and 2.40x10(8) L.mol(-1) at 291, 298, and 313 K, respectively. The thermodynamic parameters of enthalpy change DeltaH(theta), entropy change DeltaS(theta), and free energy change DeltaGtheta were -0.90 kJ.mol(-1), 157.38 J.mol(-1).K(-1), and -47.80 kJ.mol(-1) (298 K), respectively. The association constant (KA) and the number of binding sites (n) were 7.64x10(7) L.mol(-1) and 46.55 at higher concentration of MNPs, and 1.35x10(6) L.mol(-1) and 284.74 at lower concentration of MNPs. The analysis results suggested that the interaction was spontaneous and the electrostatic interactions played key roles in the reaction process. In addition, the Raman and CD spectra proved secondary structure alteration of BSA in the presence of MNPs.

Automated Detection of Fingerprint Traces of High Explosives Using Ultraviolet Raman Spectroscopy

Ultraviolet (UV) resonance Raman spectroscopy is a promising technique for the detection of trace explosives. For real-world applications, it is necessary to develop data evaluation algorithms that automatically recognize the spectral features of explosives in a sample spectrum. We have developed a robust algorithm that can tolerate high levels of fluorescence background. We successfully demonstrated the detection of traces of ANFO and TNT explosives at surface coverage levels of 55 microg/cm(2) in a blind test experiment. The sensitivity and selectivity is discussed in terms of receiver operating characteristics (ROC) curves.

Effect of titanium powder assisted surface pretreatment process on the nucleation enhancement and surface roughness of ultrananocrystalline diamond thin films

A superior, easy and single-step titanium (Ti) powder assisted surface pretreatment process is demonstrated to enhance the diamond nucleation density of ultrananocrystalline diamond (UNCD) films. It is suggested that the Ti fragments attach to silicon (Si) surface form bond with carbon at a faster rate and therefore facilitates the diamond nucleation. The formation of smaller diamond clusters with higher nucleation density on Ti mixed nanodiamond powder pretreated Si substrate is found to be the main reason for smooth UNCD film surface in comparison to the conventional surface pretreatment by only nanodiamond powder ultrasonic process. The X-ray photoelectron spectroscopic study ascertains the absence of SiC on the Si surface, which suggests that the pits, defects and Ti fragments on the Si surface are the nucleation centers to diamond crystal formation. The glancing-incidence X-ray diffraction measurements from 100 nm thick UNCD films evidently show reflections from diamond crystal planes, suggesting it to be an alternative powerful technique to identify diamond phase of UNCD thin films in the absence of ultra-violet Raman spectroscopy, near-edge X-ray absorption fine structure and transmission electron microscopy techniques.